Shoulder implant prostheses generally consist of a "head" portion of the implant which articulates with the natural or artificial glenoid surface, and a "stem" portion of the implant which provides fixation within the humeral canal. Early shoulder implants were unitary structures, being an integral structure combining the stem and the head. A device of this type is depicted in FIG. 1. Shoulder implant designs become modular in the late 1980's. A modular device generally exemplary of these prosthesis is shown in FIG. 2. These modular implants were characterized in that the head independently mated via "taper-lock" connection to the humeral stem. This modularity increased the options for the surgeon by offering significantly more sizes of heads, e.g. ten to twenty heads instead of the three to six heads available in earlier non-modular prostheses and more stems, e.g. five to ten instead of the two to four sizes of stems available in non-modular prostheses.
This modularity decreased the actual number of parts required to meet the various requirements the surgeon might face in installing such prosthetic devices. For example, with five modular stems and ten modular heads the surgeon had flay size options with only fifteen parts. Conversely, a non-modular system offering the same choices would require fifty prostheses.
Modular taper-lock connections of the head to the stem are well-known in the art[vary slightly in design]. In general, taper-lock connections comprise a tapered pin, e.g. a frustoconical pin, and a tapered receptacle that is configured to receive the pin generally but wherein there is a difference in the taper of the pin and the taper of the of the receptacle such that upon substantially full insertion the tapered pin locks into the receptacle, thereby preventing rotation of the pin in the receptacle. Such a structure is used in the present invention. It will be immediately apparent that the pin and the receptacle, respectively, can be formed in or as part of either the head or the stem, the two forms of taper lock being perfectly equivalent.
The heads of all prior art modular shoulder prostheses have incorporated a sphere or semi-spherical geometry with a male or female taper which is disposed at an angle of 90 degrees relative to the articulating surface of the head and an angle of 90 degrees relative to the undersurface of the head. This results in a fixed position of the head relative to the stem placement within the humeral canal. Once the humeral canal is prepared to accept the stem, the head is locked into the stem resulting in the fixed position. The only way to change the head position thereafter is to re-prepare the humeral canal for repositioning of the stem. This rearrangement is usually not possible due to anatomical restraints.
A principal feature of the present invention is that a shoulder prosthesis is provided that permits angular adjustment of the head relative to the stem.